Plasma markers of endothelial inflammation, PBMC expression profiles, and high-resolution cardiac MRI will also be studied in age and gender matched controls to define normal ranges and variability for each of these novel assessments. Comparison of these results to PAH subjects at baseline will be used to determine the degree to which these investigative tests distinguish PAH patients from healthy subjects. Likewise, baseline clinical evaluations of PAH subjects will be used to examine whether any novel test (inflammatory markers, or cardiac MRI), accurately classifies patients according to their disease severity. In addition, these tests will be investigated prospectively for their ability to predict PAH disease progression. Disease progression will be defined prospectively as a decrease in the 6-minute walk distance of greater than or equal to10% from baseline or clinical worsening requiring an escalation in therapy, hospitalization due to right heart failure, transplantation or death.